1. Field of the Invention
The present invention relates to a belt fixing device which fixes an unfixed toner held on a recording medium such as paper or an OHP sheet based on print data and an image forming apparatus provided with the belt fixing device.
2. Description of the Related Art
In an image forming apparatus provided with a belt fixing device such as a laser beam printer, it is essential to stably drive a belt for a long period of time.
As for a conventional technique, there is a case in which an article in which a porous resin film is laminated on a surface of a porous resin fiber woven fabric or a porous resin fiber woven fabric is used as a low friction sheet and silicon oil is used as a lubricant (see JP-A-2001-228731). However, since a void in a pressed portion of a porous resin is crushed and the lubricant is pushed out, a lean state of the lubricant is generated. Therefore, a load torque is increased in proportion to a size of an area of the portion in which the lubricant is lean. Ordinarily, most fibers are non-porous resin fibers. In order to allow them to be porous, there is a method in that they are allowed to be a foamed body or they are biaxially stretched since they do not become porous by monoaxial stretching. However, in any of such cases as described above, at the time of forming the fibers, they are apt to be cut starting from the void (crack) and, accordingly, productivity is deteriorated. For this account, there is a problem in that the porous resin fibers appear to be higher in cost than ordinary non-porous resin fibers. Further, there is a problem in that the silicon oil evaporates a low molecular weight siloxane component under a high temperature.
On the other hand, there is a case in which a sheet-like sliding material comprising a porous-structure material or the porous-structure material laminate-bonded with a porous film on a sliding face side and a deformation preventive film laminate-bonded on a non-sliding face side is used while silicone oil is used as the lubricant (see JP-A-2003-191389). However, the sheet-like sliding material laminate-bonded with the deformation preventive film becomes higher in cost by such laminate-bonding. Further, since the silicone oil is high in flowability, it tends to be diffused into other portions through permeation unless a deformation preventive film concurrently serving as an oil barrier is provided. When the silicon oil is diffused, it may contaminate other devices or deplete the lubricant on the sliding face to cause an increase of the load torque. Still further, there is a problem in that the low molecular weight siloxane component is evaporated under a high temperature.
One of technical problems concerning the belt fixing device is to stably drive the belt in a long period of time. When a frictional force between the belt and a sliding layer is high, the load torque becomes large to invite a breakage of a sliding portion or damage a surface of a roller. When an effective contact area ratio of a pressed portion of the sliding layer becomes large, the load torque tends to be increased. Further, when an oil holding ratio of the sliding sheet is small, there is a problem in that the lubricant is seeped out and enter between the roller and the belt to cause a slip of the belt, or the lubricant is depleted in the sliding portion in a long-hour driving to cause a sharp increase of the load torque. When the belt is slipped, a transportation speed of sheets is reduced to cause a problem such as folding of paper or disturbance of an image.
Further, since the sliding sheet or the lubricant is subjected to a high temperature for a long period of time, there is a possibility in that thermal degradation thereof is progressed. When the sliding sheet is thermally fused, it stuck to an inside of the belt to cause an increase of the load torque or emission of an offensive odor. When the lubricant is degraded by being oxidized, it is carbonized or becomes thick to increase the frictional force. Further, when the evaporated siloxane is adsorbed in a current-applied portion, there is a possibility of causing an electric hindrance.